1. Field of the Invention:
This invention relates to filter circuits, and more particularly, to a digital pulse filtering circuit for processing an input composite sync signal to determine the polarity thereof and thereby output the input composite sync signal in the determined polarity.
2. Description of Related Art:
In conventional monitors, a composite sync signal comprises all the sync signals necessary for proper operation of the monitors. FIG. 1 shows an example of the sync signals used in a monitor, which includes a vertical sync signal Ver(P) and a horizontal sync signal Hor(P). The composite sync signal can be either positive or negative in polarity. In practice, the horizontal sync signal Hor(P) is a first pulse train having a period of from 8.33 .mu.s (microsecond) to 33.33 .mu.s and a pulse width of from 1.0 .mu.s to 3.92 .mu.s; while the vertical sync signal Ver(P) is a second pulse train having a period of from 8.33 ms (millisecond) to 25 ms and a pulse width of from 0.0384 ms to 0.125 ms.
Given a composite sync signal, the vertical sync component can be obtained by filtering out all the pulses in the composite sync signal that have a pulse width less than that of the horizontal sync signal. Further, the polarity of the composite sync signal can be determined by filtering out all the pulses in the foregoing obtained vertical sync signal that have a pulse width less than that of the vertical sync signal itself, the output of this filtering process is a DC signal with a positive or negative level that indicates the polarity of the composite sync signal.
Conventionally, the two above-mentioned filtering processes are achieved by passing the composite sync signal through analog filters that are constructed with resistors and capacitors. One drawback to these analog filters is that their constituent resistors and capacitors take quite a large space on the circuit boards. Moreover, when the pulses that are to be filtered out is very large in pulse width, the analog filters require the use of large-resistance resistors and large-capacitance capacitors to implement, which take even larger space on the circuit boards and also cause the manufacturing cost to increase. Further, the cutoff frequency of these analog filters is not easy to adjust.